High-speed electronic applications require the mounting of very small components, for example decoupling capacitors, on very tight-pitch grid arrays which are compatible with Ball Grid Array (BGA) components. Examples of very tight-pitch grids include 0.8 mm by 0.8 mm pitch and 1.0 mm by 1.0 mm pitch grids among others.
Power supply decoupling challenges are emerging from designs with operating frequencies of 300 MHz or greater, integrated circuits with operating voltages below 3 volts and power demand of 15 watts or more. The higher operating frequencies, the ever shrinking lower and upper voltage limits given low operating voltages and the larger power demands all combine to demand higher performance from decoupling arrangements.
An ideal decoupling capacitor would transfer all its stored energy to a load instantly. A real capacitor has parasitics that prevent instantaneous transfer of a capacitor's stored energy. The actual operating nature of a capacitor can be modeled as an RLC equivalent circuit. For most purposes, it is possible to model the characteristics of an actual capacitor with the series combination of one capacitor, one resistor, and one inductor. The resistive, inductive, and capacitive values in this model are commonly referred to as equivalent-series-capacitance (ESC), equivalent-series-resistance (ESR), and equivalent-series-inductance (ESL).
The equivalent-series-inductance of a capacitor determines the speed of energy transfer to a load. The lower the ESL of an actual capacitor, the faster that energy can be transferred to a load. Historically, there has been a tradeoff between energy storage (capacitance) and inductance (speed of energy delivery) in a particular device. Low equivalent-series-inductance devices typically have low capacitance. Likewise, higher capacitance devices typically have higher ESLs. The equivalent-series-inductance of the capacitor has a further inductance added to it by the equivalent-series-inductance of the printed circuit board traces that connect the capacitor to the load—typically an integrated circuit. This drives a design topology that places the fastest low ESL capacitors as close to the load as possible to minimize the total equivalent-series-inductance.
The key physical characteristic determining equivalent-series-inductance of a capacitor is the size of the current loop it creates. The smaller the current loop, the lower the resulting ESL. A standard surface mount capacitor is rectangular in shape with electrical terminations on its shorter sides. There now exists versions of surface mount capacitors known as Reverse Geometry Capacitors (RGCs) which have its terminations on the longer side of its rectangular shape.
Surface mount capacitors have a size characterized by a four digit number of the form “XXYY” wherein “XX” designates the length of one pair of sides, and “YY” designates the length of the other pair of sides which have electrical terminations; and wherein both lengths are in hundredths of an inch. By way of example, a “0402” capacitor would have nominal dimensions of 0.04″ by 0.02″ and the ends with length 0.02″ would possess the solder terminals. Alternatively, a “0204” capacitor would have nominal dimensions of 0.02″ by 0.04″ and the ends with length 0.04″ would possess the solder terminals. Referring to FIG. 1, there may be seen an example of a 0402 capacitor 100 having electrodes 102, and a 0204 capacitor 110 having electrodes 102.
When the distance between the electrodes on the chip capacitor is reduced, the size of the overall current loop is reduced. Since the size of the current loop is the primary driver of equivalent-series-inductance, a 0306 capacitor with a smaller current loop has significantly lower ESL then a 0603 capacitor. Manufacturer reports indicate that the reduction in equivalent-series-inductance varies by standard size increments of the chip capacitors; however, ESL is typically reduced 60% or more with a Reverse Geometry Capacitor versus a standard chip capacitor.
Therefore, it would be desirable to provide a method of placing Reverse Geometry components, including capacitors, within tight-pitch BGAs.